The long term objective of this study is to identify and characterize different mechanisms used by the nervous system to recruit and modulate the activity of motoneurons during different behaviors. The specific aims are to contrast the recruitment schemes used to control a single muscle when it is excited by diverse sources, and to determine possible functional correlates of those schemes. The results of this work will help to resolve debate about the generality and utility of different recruitment schemes, and may lead to new understanding of the bases of movement disorders, such as spasticity, that involve abnormalities of recruitment or sensory processing. Using decerebrate cats, the axons of motoneurons supplying either the medial gastrocnemius or biceps femoris muscle will be penetrated in ventral root filaments two at a time. After the physiological makeup of each unit is determined, the sequence of recruitment of the two units will be evaluated during muscle stretch, natural and electrical activation of cutaneous afferents, and electrical stimulation of the mesencephalic locomotor region of the brainstem. Based on work from this and other laboratories, these stimulation regimes are designed to reveal purported shifts in recruitment strategy. Stimuli will also be delivered in combination, in order to test predictions that the firing of certain types of motoneurons may be inhibited in some conditions. Two principal hypotheses will be tested: 1) The activation of motoneurons within a single motor nucleus changes in certain motor tasks. 2) The organization of motor units in different tasks reflects different functional needs.